Print engines receive image data to produce a printed image. If a print engine is capable of printing colour, the received image data are converted to print data for the various process colours that are used to compose the printed colour image. The conversion may comprise a rendering step, a halftoning step and a colour profiling step or a colour matching step. This last step is part of the colour management module in the conversion unit and translates the colours as defined in the image data into colours as defined in the print engine. Besides image data the print engine also receives job control data that indicate what print engine requisites are accosted in printing the image data, such as the type of receiving material, whether the material is to be printed on both sides or not, the halftone type that is involved, whether a staple is to be applied or not, etc. The control unit of the print engine analyses the job control data to determine the operation of the print engine. The job control data are received in combination with the image data in a print job. The print job may be kept in a print queue from which it is automatically printed when the print engine is prepared to do so or from which it is printed on activation by a human operator.
Colour print engines, whether they are based on inkjet technology, electrophotography or other marking processes, appear to be sensitive to environment conditions such as temperature and humidity and their reproduction of colour is not as stable as one would like. Therefore a regular calibration of the intensity or the optical density of the process colours is necessary. The usual procedure for calibration comprises the printing of a test page with a number of patches with increasing intensity for each process colour, measuring the test page with a densitometer or a scanner, and providing data based on these measurements to the calibration means of the print engine. The calibration may depend on properties or values in the job control data, such as the class of receiving material, the halftone type that is applied etc. Therefore the calibration means keeps a set of calibration data for each value of the job control data. The control unit is configured to derive from the calibration means the calibration data that are to be used in the conversion unit for converting the image data into print data in a specific print job.
Since a calibration takes time, the productivity of a print engine decreases when a calibration procedure is performed. Therefore the number of times a calibration is done is to be limited. However, the time needed to see if a calibration is necessary is usually as long as doing the calibration. Therefore the step of checking if it is necessary to calibrate is usually omitted. In United States Patent Application US 2008/0239400 it is suggested to limit the number of calibrations by the time that has passed since the last calibration has taken place. Other possible solutions include restricting the execution of a calibration to the times when replacing a component, such as toner, ink or a photoconductor, or to remotely activate a calibration, when the calibration can be done automatically. In U.S. Pat. No. 7,609,414 calibration data are provided by a remote submitter of a print job, who may also restore the original calibration data. However, these solutions do not give a human operator of a print engine a feeling of control over the behaviour of the print engine he is supervising. Moreover, the number of calibrations that must be kept by a print engine, increases due to the growing variety in materials on which the images are printed. When calibration data for a certain material become outdated, it is unclear when these data need to be updated and whether that certain material is at hand for doing a calibration. Therefore a problem exists in establishing the time and the conditions for colour calibrations in a print engine.
An object of the present invention is to give a human operator the feeling of control over the calibration frequency of a print engine that he is supervising and at the same time to limit the calibration to pertinent values of the job control data.